Electron count dictates phase separation in Heusler alloys

Abstract

Phase separation -and conversely, the propensity for solid-solution formation -in half-Heusler (XY Z) and Heusler (XY 2 Z) compounds is suggested from first-principles electronic structure-based modeling to be strongly linked to the electronic behavior of the end-members. Alloying between distinct pairs of half-Heusler and Heusler compounds is possible at accessible processing temperatures when the two end-members are either isoelectronic or metallic. The formation of a band gap in semiconducting half-Heusler compounds is associated with significant stabilization. Attempts to create solid solutions with a semiconducting half-Heusler compound would lead to phase separation across the tie line because of the energy penalty associated with filling states in the gap. The alloying between two Heusler compounds, however, is expected even when the electronic behavior of the end-members differ -clearly demonstrating the distinction between the underlying bonding within half-Heuslers and Heuslers: half-Heuslers are well-defined intermetallics whereas Heuslers tend to behave in a manner more in line with conventional alloys. The simple proxy related to electronic structure developed here differentiates between Heusler and half-Heusler compositions that truly alloy from those that phase separate, aiding in the pursuit of reliable first-principles materials discovery.

Superconductivity–Electron Count Relationship in Heusler Phases─the Case of LiPd2Si

“…Within this simple approach, the electronic structure is assumed to be qualitatively robust toward changes in the elemental composition, and the difference between the properties thus stems from the change in the occupancy of different electronic states (i.e., the position of the Fermi level). While such a view is an obvious oversimplification, it is successful in explaining, for example, the effect of chemical doping on magnetic properties of Heusler compounds , and the stability of XYZ-XY 2 Z half Heusler–full Heusler solid solutions . The use of molecular orbital (MO) theory, or more precisely crystal orbital (CO) theory, allows one to put the relationship between VEC and the physical properties into the context of chemical bonding.…”

“…While such a view is an obvious oversimplification, it is successful in explaining, for example, the effect of chemical doping on magnetic properties of Heusler compounds 4 , 5 and the stability of XYZ-XY 2 Z half Heusler–full Heusler solid solutions. 25 The use of molecular orbital (MO) theory, or more precisely crystal orbital (CO) theory, allows one to put the relationship between VEC and the physical properties into the context of chemical bonding. An example of this approach is presented in the recent literature, 4 where it is explained that the exceptional stability and semiconducting properties of VEC = 8 and VEC = 18 half Heusler compounds by the closed shell configuration found for that electron count.…”